1. Field of the Invention
The present invention relates to a nitride based semiconductor device and a process for preparing the same, and more particularly to a nitride based semiconductor device having a GaN layer formed on a silicon substrate and a process for preparing the same.
2. Description of the Related Art
Recently, a great deal of attention has been directed to gallium nitride based semiconductors, as gallium nitride is suitable for application to photoelectric devices with short wavelength bands and high performance electronic devices. Blue and green light emitting diodes made from Group III-nitride based compound semiconductors employing gallium nitride (GaN) were commercialized in the late 1990s. White light emitting diodes are also manufactured from GaN based compound semiconductors, and recently have succeeded in commercialization, thus rapidly increasing in demand thereof. In order to manufacture such nitride based semiconductor light emitting devices, a technique for growing a high quality nitride based single crystal is essential. However, there is a problem that no general substrate for growing nitride based single crystals, which matches with a lattice constant and thermal expansion coefficient of the nitride based single crystals, is available.
Usually, nitride based single crystals are grown on heterologous substrates such as sapphire (Al2O3) substrate or silicone carbide (SiC) substrate using gas-phase growth methods such as Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) or Molecular Beam Epitaxy (MBE). However, single crystal sapphire or SiC substrates are expensive and the size thereof is strictly limited to a range of about 2 to 3 inches, thus being unsuitable for mass production.
Meanwhile, where a GaN film is grown on the silicone substrate, it is possible to manufacture a large diameter substrate, resulting in reduction of production costs, to employ conventional silicone device manufacturing methods and apparatuses and to realize monolithic integration of GaN based devices on the silicone (Si) substrate, and thereby it is possible to realize a combination of silicone devices and GaN based devices. Therefore, there remains a need in the art for use of the Si substrate that is most generally used as the substrate in semiconductor industry, other than light emitting devices.
However, due to lattice mismatch resulting from differences in lattice constants between the Si substrate and GaN single crystals, it is difficult to directly grow the single crystal GaN layer on the Si substrate. Further, since there is a difference of about 35% between the thermal expansion coefficients of GaN and Si, when the GaN film is grown directly on the silicone substrate followed by cooling to room temperature, it leads to generation of cracks due to remaining stress in the GaN film. In addition, since the silicone substrate surface exhibits poor wettability of Ga and GaN is not thermodynamically stable as compared to silicone nitride (Si3N4), direct growth of the GaN film on the silicone substrate may result in an amorphous Si3N4 film on the surface of the exposed silicone substrate.
Two methods of solving such problems are available. First is to form a low temperature AlN buffer layer on the Si substrate, followed by formation of a GaN epitaxial layer. Second is to form a buffer structure having a multilayer combination of a low temperature AlN buffer layer and AlGaN/GaN on the Si substrate followed by formation of a GaN epitaxial layer thereon. However, even though the GaN layer was formed in such a manner, it fails to fundamentally improve the problems associated with lattice mismatch, and thus it is difficult to easily grow the GaN epitaxial layer on the buffer layer or buffer structure and there suffers from cracking. In particular, when directly forming the GaN layer on the low temperature AlN buffer layer, the GaN layer grows 3-dimensionally rather than two-dimensionally, thus producing island growth leading to poor surface roughness and lowering carrier mobility.
FIG. 1 is a cross-sectional view of a conventional nitride based semiconductor device having a GaN layer formed on a Si substrate. The conventional nitride based semiconductor device using the Si substrate shown in FIG. 1 is prepared by vapor depositing a low temperature AlN buffer layer 12 on a Si substrate 11 at direction (111) using conventional methods at a temperature of 500 to 700° C., forming an AlGaN/GaN intermediate layer 13 having a multilayer structure, and then growing an undoped GaN layer 14 thereon. This low temperature AlN buffer layer 12 and AlGaN/GaN intermediate layer 13 form a buffer structure that reduces the lattice mismatch between the lower Si substrate 11 and upper GaN layer 14.
However, since the lattice mismatch between Si and GaN is very high, i.e., about 20%, cracks occurring due to lattice mismatch, in spite of forming such a buffer structure, are still present, and complexity increases which requires formation of an AlGaN/GaN intermediate layer 13 having a multilayer structure. Therefore, there is required a method capable of forming a good quality GaN layer on the Si substrate in a more simplified process.